A variety of processes and solutions have heretofore been used or proposed for use to deposit a dark or substantially black nickel deposit on various conductive substrates. Such so-called black nickel deposits are particularly suitable for various decorative purposes as well as to promote absorption of radiant energy such as in solar heating systems, and the like. Typical of such prior art techniques for depositing a black coating or black nickel deposit on metallic substrates are those disclosed in U.S. Pat. Nos. 2,679,475; 2,844,530; 3,127,279; 3,681,211 and 3,753,873.
A continuing problem associated with such prior art techniques has been the difficulty in controlling the composition and process to consistently achieve substantially black coatings which are adherent to the substrate, which provide for improved corrosion resistance, and which are receptive to receiving a clear lacquer or other siccative finish coating.
A recent improvement in an electrolyte composition and process for overcoming many of the problems and disadvantages associated with prior art techniques for electrodepositing dark nickel platings is disclosed in United States Patent Application Ser. No. 71,610 filed Aug. 31, 1979, now U.S. Pat. No. 4,244,790. It has been found in some instances, however, that the aforementioned improved electrolyte produces non-uniformity in the dark nickel deposit in high current density areas and rainbow colors and/or skip plate in low current density areas of parts of complex configuration during commercial rack plating operations.
The present invention provides for a still further improvement in the art of dark or black nickel plating of parts of complex configuration by providing an electrolyte and process which achieves an increase in the rate of electrodeposition over a broad range of current densities, pH, bath concentration and temperature and is adaptable for use on a variety of different conductive substrates achieving consistent, substantially uniform black nickel deposits in low, intermediate as well as high current density areas. The dark nickel deposits are further characterized by their good corrosion resistance, adhesion and receptivity to a variety of clear lacquer finish coats.